Mechanics

During a battle, a Trainer can use their Key Stone (held in the Mega RingXY/Mega BraceletORAS for the player character) to Mega Evolve a Pokémon that holds its corresponding Mega Stone before it attacks. Pokémon only remain Mega Evolved during battles and Pokémon Contest Spectactulars, and will revert to their original state once the battle has ended or if they faint. Every Pokémon that can undergo Mega Evolution has a corresponding Mega Stone that is named after it, except for Rayquaza, which instead needs to know the move Dragon Ascent to Mega Evolve. Some Pokémon have multiple different Mega Stones for their species.

Mega Stones cannot be transferred to or from the correct holder during battle: Thief and Covet do not take the Mega Stone, Knock Off does not remove the Mega Stone, Trick and Switcheroo fail, Fling and Bestow fail, and Symbiosis does not activate. Embargo and Magic Room also cannot prevent the use of Mega Stones to Mega Evolve Pokémon. Mega Stones are not consumed upon use, but only one Pokémon can Mega Evolve per battle.

Availability

In Pokémon X and Y, all Mega Stones not directly given or sold to the player can only be found between 8 and 8:59 p.m. after Professor Sycamore has upgraded the player's Mega Ring to detect them. These Mega Stones do not appear as item balls in the overworld; rather, they appear as sparkles on the ground.

The stone for Blaziken, the Blazikenite, cannot be found through normal gameplay in X and Y. However, it can be obtained via special distributions and trading. The stone for Diancie, the Diancite, also cannot be obtained through normal gameplay in Omega Ruby and Alpha Sapphire. Instead, the stone can be obtained by transferring or trading Diancie from Pokémon X and Y to Pokémon Omega Ruby and Alpha Sapphire, activating an event in Omega Ruby and Alpha Sapphire that will allow the Diancite to be obtained.

Mega Stones made their first physical appearance in Mega Evolution Special I. The seen Mega Stones were Alain's Charizardite X, Astrid's Absolite, Remo's Garchompite, Siebold's Blastoisinite, and a Gyaradosite found by Alain from ancient ruins. In addition, a Lucarionite was seen in a painting shown to Alain by Professor Sycamore in a flashback, featuring the first ever Mega Evolution. Astrid's Absolite made a brief reappearance during the opening credits of Diancie and the Cocoon of Destruction.

A Lucarionite made an appearance in The Cave of Trials!, where it was found by Korrina in a cave near Geosenge Town. After battling a Blaziken guarding the Lucarionite—which was afterward revealed to belong to Korrina's grandfather, Gurkinn—Korrina obtained the Mega Stone and used it to Mega Evolve her Lucario. The uncut Lucarionite was later set in an armband by McGinty, allowing Lucario to wear the Mega Stone.

Another Lucarionite appeared in The Aura Storm! under the ownership of Gurkinn. Much like Korrina's Lucario, Gurkinn's Lucario also wears its Lucarionite in an armband on its left wrist.

Steven's ShinyMetagross's Metagrossite appeared in Mega Evolution Special II, wearing it as a large bracelet on its right foreleg. An Ampharosite was also seen in a flashback in the same episode, having been found by Lysandre before Alain could get his hands on it.

Another Garchompite, belonging to Professor Sycamore and his Garchomp, appeared in Garchomp's Mega Bond!. Garchomp wears her Mega Stone as a choker around her neck.

Several different Mega Stones appeared in Mega Evolution Special IV. The seen Mega Stones include a Venusaurite attached to the tree on a Venusaur's back, an Alakazite tied onto one of Alakazam's spoons, and Malva's Houndoominite, which is attached to a collar her Houndoom wears around its neck.

In Pokémon Origins

Charizardite X (left) and a Key Stone (right) in Pokémon Origins

In the Pokémon Origins episode File 2: Cubone, Mr. Fuji gave Red a Mega Stone and a Key Stone, although he did not tell Red what they truly were, saying that he would find out soon enough. In File 4: Charizard, during Red's battle against Mewtwo, one of the stones was revealed to be a Charizardite X from Kalos. Together, it and the Key Stone triggered Red's Charizard's Mega Evolution, allowing it to Mega Evolve into Mega Charizard X and weaken Mewtwo enough to be caught.

In the Pokémon Adventures manga

In the X & Y chapter, due to Mega Evolution being a relatively unknown phenomenon, the Mega Stone concept gradually developed. Trevor was not aware of the Mega Stone that X's Kanga possessed, until They Have a Flare for a Li'l Kanga-Napping when he found it in Kanga's pouch. In What Does Charmander Do When It Dozes?, Trevor and X proved that Kanga's Mega Stone had no effect when taken out of Kanga's pouch and placed with X's Chespin. In Overthrowing a Tyrunt, Trevor and X became aware of different Mega Stones and their names when the Team Flare Grunt they were fighting mentioned the Kangaskanite and Manectite explicitly. Trevor and X combined their skills to defeat the grunt in battle and steal a list of Mega Stones that he had supposedly compiled.

Korrina's Lucario possesses a Lucarionite, allowing it to Mega Evolve into Mega Lucario.

Lysandre's Gyarados possesses a Gyaradosite, which a Team Flare grunt successfully dispatched to Malva.

X's Élec possesses a Manectite, which is kept in a patch of fur on his chest. Élec was motivated to seek its Mega Stone after witnessing Kanga and Li'l Kanga Mega Evolve during a battle in the Prism Tower.

X's Garma possesses a Gengarite, which is kept in a patch of fur under its jaw.

Tierno and Shauna found multiple Mega Stones while looking for a Pinsirite and Absolite for X's Rute and Y's Solsol. After some searching, they successfully found the correct Mega Stones and gave them to X and Y.

X's Salamè possesses a Charizardite X, which he keeps on the tip of his tail, hidden inside of tail's flame. Blue's Charizard possesses a Charizardite Y, which is also kept hidden inside of the flame on the tip of his tail. Both Charizard combined in their respective Mega Evolutions to defeat Xerosic, who claimed that Team Flare found no proof that a Charizard Mega Evolves, let alone into two different forms.

Blaine possesses both a Mewtwonite Y and Mewtwonite X, which he can use to Mega Evolve Mewtwo into either of its Mega Evolutions.

In Omega Alpha Adventure 3, Zinnia used her Salamencite to Mega Evolve her Salamence during a battle against Ruby. Using Latios's Latiosite, Ruby Mega Evolved the Eon Pokémon into Mega Latios and defeated Mega Salamence.

In PAORAS11, Red and Green were revealed to have respectively obtained the Venusaurite and Blastoisinite, sometime before following Maxie and Archie, who had obtained the Cameruptite and Sharpedonite, respectively. The two pairs of Trainers used the Mega Stones with their respective Pokémon to battle each other.

In PAORAS20, Giovanni was revealed to have obtained a Beedrillite, using it to Mega Evolve his Beedrill as part of his explanation that the meteoroid threatening to destroy the world may be housing a Pokémon inside of it. The Meteor Villageelder later offered Sapphire a Gardevoirite and Galladite, commenting how matching her Gallade and Ruby's Gardevoir are. Ruby and Sapphire later used these Mega Stones to Mega Evolve Rara and Kirly during a brief battle against each other.

In PAORAS21, after destroying the Grand Meteor Delta, Ruby has Hoopa find a Diancite for Diancie to use in the Pokémon Contest Spectacular, allowing Ruby to Mega Evolve Diancie for his and Sapphire's performance.